Method of drying slip cast materials and the like



' April 1957 L. R. ARMSTRONG ETAL 2,789,338

METHOD O DRYING SLIP CAST MATERIALS AND THE LIKE Filed Sept. 25, 1953' INVENTORS. LELAND R A RMSTRONG 50W RD C. HENR A 515 MM 8 (0.4. 02 BY K05527- roams-s ike United rates @ttent O NIETHOD OF DRYING SLIP CAST MATERIALS AND THE LIKE Leland R. Armstrong, Washington, Edward C. Henry, State College, and James M. Lambie and Robert A. Youngs, Washington, Pa., assignors to hrndlay Clay Products Company, Washington, Pa., a corporation of Pennsylvania Application September 25, 1953, Serial No. 382,303 2 Claims. (Cl. 25-156) This invention relates to a novel method of controlling the drying of large or complicated ceramic shapes which are otherwise difficult to dry without cracking.

Tank blocks for glass furnaces, and other large clay pieces used'in the manufacture of glass, are commonly made by the slip cast process. The slip is prepared by mixing clay and grog with water and deflocculants and the ceramic shape is formed by pouring the preparedslip, into a plaster mold of the proper design. The slip in immediate contact with the plaster loses water rapidly and a wall of semi-rigid clay is formed. This wall builds up at a diminishing rate because the clay deposited has small pores. As soon as a sufficient thickness of wall has been built up to prevent distortion, the mold is re: moved and drying proceeds from the five sides that are exposed to the air, and water moves from the bottom side, which is in contact with the plaster base. At this time, the central mass of the block is in a fluid or semi fluid state. The top of the block is somewhat damper than the sides, because the rate of drying has been reduced by a covering of moistened burlap. At the time the mold is removed, there is a rather steep moisture gradient from the center to the sides and particularly from the center to the corners and edges.

In drying, the clay shrinks and if the piece is not to crack, the'shrinkage of all portions of the blocks must take'place at as near the same time as possible. This means that there must be a low or nearly level moisture gradient from the center to the various parts of the block. Water must be absorbed from the bottom by the plaster at somewhere near the same rate that it evaporates fr'orn'the other surfaces of the block. This is accomplished by holding the block in a nearly saturated atmos phere and at the same time warming up the block. Warming' the block decreases the viscosity of the Water and soincreases the rate of flow from the center to the surface. This has heretofore commonly been done by covering the block during drying, with burlap, either wet or dry, and possibly by the addition of a waterimpervious paper at the corners and edges. During this time, theblock is kept in a warm room 70 to 90 F. The burlap and water impervious paper act to hold the air which is between the block and the burlap at some humidity. By this process, the blocks remain on the base from seven to nine days, when it is judged that the center is entirely rigid. The blocks are then removed from the base, again wrapped with burlap and stood on end in. a similarly-warm room to dry, Where they must remain for forty-five to ninety days, depending on the size of the'block. The common method of drying'large shapes as described above does not control humidity around the shape very closely at all times, but issubject to variations, due to wrapping and air movement aroundv the clay shape. For this reason, drying-must proceed slowly in low-temperature air. As carefully as thismethod of drying can be carried out, a certainamount oftlosses due to cracking of large shapes is encountered ice due'to variations in temperature and humidity that are not 'under'clo'se control.

To dry large clay shapes faster and safely, a close control of humidity and temperature must be established.

In humidity driers, which are common in the ceramic industry, the temperature and humidity of the air are controlled and'this air is circulated around the products. Accurate control of humidity and air circulation is a difficult task, even with elaborate and expensive equipment. In particular, We have encountered extensive troubleinapplying the humidity system of drying, as currently visualized, to large ceramic shapes.

We have discovered, however, that the humidity and air circulation can be controlled effectively, if empirical- 1y,- merely-bymaintaining control of the temperature, by means w'e-deseribe' below. In the usual application of our discovery,-wemake use of a tunnel dryer, but the methodis applicable in room drying also.

We herein show a structural arrangement for practicing our method in the drying of cast tank block's.

Asshown in the accompanying drawings, Figure l is a perspective view-of the box or cover for the wet castings or other wet shapes, and

Fig. 2 is a perspective schematic view showing one formof apparatus-for use-in drying the castings.

As soon as the mold is removed from the cast block 3; which may be resting on a mold bottom 4, we cover the block with a cardboard box 5'which has been made impervious-to moisture by acoating of asynthetic rubber latex such as Geon, or some similar Water-impervious substance. This box is made slightly larger than the block so that it does not touchthe block at any place.

The box referred to may be made of any convenient material which is either impervious to'water vapor or may be made sufficiently-water-vapor retardant'by painting, spraying, or other treatment. A metal box would serve .well. We have preferred to use heavy corrugated cardboard because of its cheapness and lightness of weight.

A very. small amount of water evaporatingfrom the block saturates the spacebetween the'block and the box. In order to control the humidity of the air between the box and the block and to allow escape of moisture, we cut'small holes 6 in the box. The size of the space between .the box and the block controls the amount of water at first evaporating from the block. The temperature of the surface of the block largely determines the rate'of evaporation of water from the clay surface, and thus the'rate of approach to saturation of the enclosed air space. The number and size of holes control the rate at which said water vapor leaves the space between the block and the box. In other words, by varying the sizeof the-box relative to the block, and the number of holes in the'box, and the temperature, we are able to control theescape of moisture and thus the drying of the block.

As a typical example, we have found that the optimum clearance between the block and the box should be of the-order of 1 inch to 4 inches, the size of the holes between- A inch and-1 inch, and the number of holes 10 to 20, for a block 12 x 24 x 36 inches. The ratio of the volume of controlled air space to that of the shape to be dried is generally in the range 1:3 to 2:1, but we do not wish to be held to a specific number of holes and volume relation between the block volume and controlled air space as they are interrelated to the temperature we are maintaining.

The blocks are cast on a truck 7 which can be pushed into -a heated tunnel 8. The blocks are heated from the outside throughthe boxes 5. No condensation can take place on the inside of the box through slight temperature greases 3 changes, becausethe "boxis always warmer than the surface of'the block.

The circulation or movement of air through the holes 6.is preferably by convection current, the saturated .or moistened air from within the box of -course moving into the tunnel chamber, through the holes 6. Thetunnol walls may be of somewhat porous fibrous material such as some well-known fiber boards through which therecan be breathing of air, in which case the moisture can simply seen through these walls. Of course. if more solid tunnel Walls are used, small holes oropenings could be provided for slow escape of moistened air fromthe tunnel.

The humidity ofthe air between the box and the block is-thus controlled empirically so that the block drieswithout cracking and in a minimum time.

Instead of the tunnel 8, alargehood could, of course, be placed over the casting andheati-supplied thereon'either by steampipes 9 or electrical'resistors, or e q gbygentlc movement of heated air from 'outside of the hood.

Also, the number and. size-of the holes 6 will be'varied in accordance with the amount of moisture or water that has to beslowly removed from a ceramic body, or according tothe size of the body, orthe amount of heat employed. These variables will be madeto conform to'the requirement that a slim cast block 12 x 24 x "36'inches for example, will be dried in about seven daysto a point at which they may be removed from the box andthe drying safely 'completedin open air, or in another section ofthe tunnel or another hood.

We hold the temperature of the tunnel at 130 to 140 F. and in'seven days the blocks are dry enough'to be removed from the box and dried safely in open 'airor in another section of the tunnel. Thus, the blocks can be dried safely, with less'handling than by the conventional method. Furthermore, they may be placed in the kiln l3 to 15 da s after casting. by our method of drying, as opposed to 45 to 90 days by the usual method as described ahove, and blocks so dried are without internal cracks, which we frequently have in'ordinary drving due to the delayed shrinka e of the central mass. "These internal'cracks arefreouentlynot discernible until the b ock has been in use for some time in class furnace. The temperatures specified are those usually used, but We do not to be held to this particular tem erature limitation. With awlower tcmperaturewe could et good results over a lon er time. and at sons what higher tem eratures, We could szet good results. but the plasterbases would be ruined by temperatures that are too. high.

We 'have'found that drafts in our drying rooms have caused some blocksto crack While other blocks remained intact, although the method of covering might have been uniform. The woven texture of the burlap permits change of air under the burlap and makes drying of ablock 'm a very slight draft uneven andunpredictable. The movement of air across theface of a bloclcisimpossibleito measure but is a very importantfactor. We'have found that on Sundays, when there is no passing along an aisle in the drying room, burlap covered blocks will-dry more slowly than they do on working days when the 'air is stirred due to frequent passage of men pastthe blocks. In our tunnels, the blocks are covered with a tight box, the water from the block is gradually dissipated-through the controlled holes in our boxes, and air movement across'the sides of the block is reduced and uniform.

It is necessary to cool the blocks to'room temperature before loading them into the'kiln. Since it is dangerous to cool'a large piece too rapidly, the blocks "are passed through a tunnel which allows them 'to cool "gradually from tunnel temperature to room temperature.

The success of our method is shown dramatically-by the fact that with thismethod, we have been ableto reduce the loss on dryingordinary tank blocks'from to *to practical.l-y"0% We now describe our method of drying pieces too largeto beplaced on trucks and put inthe tunnel dryer.

By the use of the method we have discovered, the basic principles of which have been described above, we have been able also to make large pieces weighing 3500 lbs. without loss or cracking which previously had resulted in 50 to loss, and even to dry successfully, certain pieces we had previously been completely unable to dry without cracking by any expedient then known to us. Such large pieces are not run through block drying tunnels but are kept at elevated temperatures in "small insulated chambers. Each large piece is coveredby a Geon-coated box with controlled openings, .and then placed in an insulated heated chamber corresponding to the'tunnel.

We have, in etfect,,a humidity system of drying, precisely controllable without the necessity of separately controlling the humidity and the velocityof large volumes of .air. Equally important is the fact that, whether in the tunnel or in the individual chamber, we can keep each piece .surrounded by air which has practically no movement, while maintaining that air at a temperature and humidity which would be extremely uncomfortable and perhaps even hazardous to the workmen in the factory. By our method, the free air in the room containingthe drying chambers or the tunnels need not have unusual temperature or humidity.

The heat, whether supplied by electrical resistors, or steam pipes, or heated air, can, of course, be controlled by thermostats located in the tunnel, and there can be variations eflfected in temperaturcduring the drying period; or, with a long tunnel, the temperature can be regulated to various degrees at certain locations along the track. For example, we may subject the blockto successively cooler temperatures during the treating period.

We claim as our invention:

1. The method which comprises forming a slip cast ing, and the like, in a mold, removing the mold walls when the face areas of the casting have .become set sufiiciently to maintain the shape of the casting, supporting the casting on a base member, loosely-covering the casting With a box whose Walls are in spaced relation to said'areas and which has holes for the circulation of .air through the box walls, placing the box and castingin an enclosure whose walls are pervious to moisture but pre ventfrce flow of air therethrough, and which is of sub stantially greater height and horizontal dimensions than the exterior of the box, supplying .heat to the exterior surface of the box and through said holes, at atemper-ature not in excess of twice average room temperature, and for a period sufficient to effect removal of such quantity of the moisture from the casting .as will resultiin approximately uniform moisture contentthroughout the casting, thereafter drying the casting more rapidly, to aecondition suitable for firing, and thenfiring it.

-2. .The method which comprises forming a slip casting and the like, in amold, removing the mold walls when the face areas of the casting have become set sufficiently -to maintain the shape ofthe casting, supporting the casting on a base member, loosely-covering the casting with a box whose walls are in spaced relation to said areasand which has holes for the circulation of air through the box walls, passing the box and the casting through a tunnel of substantially greater height and width than the exterior of the box, and effecting circulation of heated'air through the said holes, the tunnel walls being of a character to prevent access of substantial quantitles of air into it from the exterior thereof,:but pervious to the escape of moisture from within the enclosure, varying the temperature in'the tunnel in'relation'to'the size-"and number of the holes, the'drying operation-being carried *on until sufficient moisture has been removed from the'casting as will result in'approximately .unlform moisture content throughout the casting, thereafter dry- References Cited in the file of this patent UNITED STATES PATENTS Simonds Feb. 6, 1917 Brain Mar. 9, 1920 Howson Jan. 2, 1923 6 Dressler Aug. 19, 1924 Rho-ads Mar. 22, 1927 McCoy Oct. 14, 1930 Wagner Nov. 17, 1931 Bower Feb. 17, 1942 Bellezza May 23, 1950 Cremer Jan. 20, 1953 

